Smartphone speakers are bad enough in a quiet room, and basically worthless with any ambient noise. An empty pitcher or vase though can amplify and direct the noise surprisingly well.

Similar to the alarm clock in a cup trick, but on a larger scale, this can boost your phone's volume just enough to use it in a pinch if you're entertaining friends, or if you want to listen to music or radio in the shower. If your vessel of choice can lie flat, you can also try turning it on its side to direct the sound in one direction.